On 12-Aug-2003, Tom Watson wrote:

D = 0.1563wl^4
_____________

Ebh^3

Whe

D = deflection (in inches)
w = load per lineal inch of span
l = span (length)
E = modulus of elasticity
b = base (width)
h = depth (thickness)

Actually, this is likely to be a lower bound to the deflection. Since the
sheet is significantly wide and fairly flexible, there will be some sagging
resulting from bending in both directions (plus or minus anticlastic curvature
effects). The deflection of a plate is a lot more complex than that of a beam.

You'll never get better than a rough estimate from basic elastic theory, since
wood in general (including plywood) is not a nice isotropic material.

You really want to reinforce this plywood sheet, as so many have pointed out.
Wood will be more weight efficient that steel angles. Tables are made like
tables for a good reason. The torsion box is a nice alternative.

Mike